>> Earth, a unique planet,
restless and dynamic.
Continents shift and clash,
volcanoes erupt,
glaciers grow and recede--
titanic forces that are
constantly at work,
leaving a trail of geological
mysteries behind.
One of these mysteries
is centered here
in the Grand Canyon in Arizona.
Close to a billion tons of rock
have been carved out
of the ground.
The canyon left behind
could hold all the river water
on Earth and still be less
than half full.
For more than a century,
scientists have debated
how and when this vast chasm
was created.
And now geologists are
uncovering fresh evidence
of how the Grand Canyon fits
into the ever-evolving story
of "How the Earth Was Made." S02x01 Grand Canyon Original Air Date on November 24, 2009
-- Sync, corrected by elderman -- -- for www.addic7ed.com --
The Grand Canyon...
one of America's most
spectacular natural wonders...
A canyon 18 miles across
at its widest point...
277 miles long...
and more than a mile deep.
It is so vast that it can even
be seen from space.
Although Hell's Canyon in Idaho
is almost half a mile deeper,
and Australia's Capertee Valley
is nearly a mile wider,
the Grand Canyon remains
the most famous of them all.
And it also holds one of
geology's greatest mysteries--
Just how did the Colorado River,
only a tenth the size
of the Mississippi,
form such a large canyon?
The answer has eluded scientists
for more than a century,
because many of the clues
they normally rely on
have been swept away
by the river's water
over millions of years
or buried by landslides
or destroyed by volcanoes.
>> It seems like we should
understand perfectly
how the Grand Canyon formed.
The problem is, we've lost
a tremendous amount of evidence.
It's like a murder mystery where
most of the evidence is lost.
And so the best we can do
is piece together the evidence
that we have.
>> Even so, slowly but surely,
this geological icon is giving
up its most ancient secrets.
The canyon's richly colored
layers offer scientists
one of the most complete
geological records on Earth.
>> The first concept you have to
get your mind around as you're
thinking about the Grand Canyon
is that the stories told by
the rocks are exceedingly old,
millions and billions of years.
>> Karlstrom and his team
are setting out on a grueling
geology field trip
along the Colorado River.
It won't be an easy ride,
because this 1,450-mile-long
river packs a punch.
More than 800 million gallons
of water can flow down
the Colorado every hour...
more water every second
than the average U.S. household
uses in a year.
Karlstrom is investigating
the ancient history of the land
that was here before
the Grand Canyon even existed.
And for that, he needs
to identify its oldest rocks.
He is following in the footsteps
of pioneer explorer
John Wesley Powell.
In 1869, he was the first man to
successfully ride the Colorado
through the entire length
of the canyon.
>> All of us who work
in the canyon as scientists
admire John Wesley Powell
immensely for his pioneer
and scientific exploration
of the Grand Canyon,
And the questions that he framed
are still questions
that we work on today.
>> One of Powell's discoveries
was these intimidating black
rocks at the very base
of the canyon.
>> Well, we're deep in the Grand
Canyon, right by the Colorado
River. You can see these
spectacular black rocks.
Actually John Wesley Powell
called them ugly black rocks
because for him, these hard
rocks made bad rapids,
and that was harder on his trip.
But for those of us who are
interested in the early history
of Grand Canyon, these rocks
are the bonanza.
>> Powell had no way of dating
these rocks, now identified
as vishnu schist.
All he could conclude
from their appearance was that
they had once been molten
deep underground, but Karlstrom
has an advantage--
modern instruments that can
accurately date the rocks
by measuring radioactive decay.
And the first step
in figuring out what happened
here in the ancient past
is to record when these rocks
were created.
>> These rocks are about
1.7 billion years old.
It's less than half
of the age of the Earth.
So we have a great story here
in the Grand Canyon of the last
almost 2 billion years
of Earth's history.
>> But Karlstrom needs
more information,
and these ugly black rocks
hold another crucial clue
to what this land looked like
before the canyon was cut.
They can tell him not only
when they were formed,
but also precisely how deep
in the Earth's crust
they were made.
These tiny stones embedded
throughout the ancient boulders
are literally jewels,
garnets that only form
under immense pressure,
the sort of pressure that's
found when layers are crushed
by the weight of millions
of tons of rock on top of them.
>> The silver-bullet clue
is the garnet.
These garnets are the key
to understanding the amount
of rock above us.
>> By analyzing the chemical
structure of the garnet,
in particular its calcium
content, investigators can
determine how much weight of
rock was crushing down upon it
at the moment it was made.
>> In simple terms,
if you analyze the garnet
and you see higher calcium
content of the garnet,
it means you're deeper
into a mountain belt,
more rocks above you.
So we take these garnets
back to the laboratory.
We cut a very thin section.
We put them under
an electron microprobe,
and the scientific result
after this analysis is that
we were 6 miles deep beneath
the surface of the peaks
which were above us,
and that's a long ways. Ha ha.
>> So, nearly 2 billion years
ago, before the canyon evolved,
ancient mountains 6 miles
above sea level stood here,
towering peaks as high
as the modern Himalayas.
Over the next 500 million years,
these mountains were worn away
by the relentless forces
of erosion. Over millennia,
the freezing and thawing of ice
cracked open the rock
of the mountain slopes.
Wind and water carried the rock
debris down towards the oceans,
leaving behind a flat
and featureless plain
with no sign at all of a canyon.
>> Geologists learn to visualize
the way that this place looked
in the past. Knowing how to read
the texture of the rock,
the kind of rock it is,
the fossils that are in it,
geologists can--
It's like a detective story.
You can uncover what
this place looked like
billions of years ago.
>> This is now desert country,
more than 300 miles inland.
And yet these shells
encased in solid rocks...
are ocean fossils.
>> In this one cliff,
you can find fossil shells
that look like you'd pick up
on the seashore today.
They die, they fall to
the bottom of the sea floor,
and they get trapped
and die in the mud
at the bottom of the ocean
at the time that they're
being deposited.
>> Shells like these come
from shallow, tropical waters,
an inland sea that first arrived
here half a billion years ago
and covered the flat,
low-lying plain.
But that did not happen
just once. Many different layers
in the walls of the Grand Canyon
tell Karlstrom that over
hundreds of millions of years,
this land has been submerged
by the sea not just once,
but at least 8 times.
The last time this part
of Arizona was under the sea
was around 80 million years ago.
>> As we go higher in the layers
in the Grand Canyon, we have
different-aged seas, which are
depositing different kinds of
rocks, different environments,
different fossils that live
at the different times.
This chapter of seas coming in
and seas going out is itself
hundreds of millions of years.
>> Each sea deposited
different types of material that
hardened to become solid rock.
Some sediment was sand that
gecame buff-colored sandstone,
some was mud that hardened
into darker shale,
while the calcified remains
of marine organisms were crushed
into light-colored limestone.
And yet the dominant color
is red.
That comes from iron
locked within all the rocks.
Over millions of years,
the iron rusts into
a distinctive red hue.
For the geology detectives,
descending into the Grand Canyon
is like traveling back itime.
The calcium content inside
garnet gemstones reveals
that nearly 2 billion years ago,
mountains the size
pf Mt. Everest stood
where the Grand Canyon is now.
Sea fossils exposed
in the cliffs show that as late
as 500 million years ago,
the land was the muddy bottom
of an ancient inland sea.
The next puzzle for geologists
is uncovering which awesome
forces transformed that
unremarkable flat land
into this breathtaking
natural wonder of the world.
>> From 1.7 billion years ago
to 70 million years ago,
the landscape of western Arizona
has undergone a series
of extraordinary changes.
Ancient mountains have given way
to prehistoric seas
which have withdrawn to reveal
a low-lying flat plain
stretching as far
as the eye can see.
The magnificent gorge
of the Grand Canyon
does not yet exist
but over the next 20 million
years, this landscape was
to undergo immense changes
that would create a unique
high plateau and set the scene
for the formation of the canyon.
Ancient fossils of sea animals
tell geologists that this land
was once under the waves
of an inland ocean, but that
leads to yet another mystery.
The investigation needs
to figure out why these undersea
rocks are now high in the air,
thousands of feet
above sea level.
>> It's surprising to go up
a mile above sea level,
and you find a clam shell
or what looks like a clam shell,
and you say, "that's what I see
"when I go down to the ocean."
"so why is it here,
a mile above sea level?"
>> It's clear that this region
underwent a type of geological
disturbance that pushed up
the entire seabed.
Geologists discovered
in the 1960's that collisions
between separate plates
of the Earth's crust could
force land up into the air.
It happens all over the globe
and usually deforms the land
into tilted mountain ranges.
But this Arizona uplift
was unique.
>> After all the flat layers
are deposited at sea level,
there was a major uplift event
called the Laramide Orogeny,
which lifted these rocks
without tilting them,
still flat, lifted them up
to high elevation.
>> Because the land rose
straight up, like being
in an elevator, it formed
a high, smooth plateau.
The sea that had been there
drained back toward
the northeast, but as of yet,
there was no Grand Canyon.
The Colorado River, the force
that cut the canyon from
the rock, had yet to arrive.
>> Geologists, from the very
early days, from the late 1800's
are quite comfortable
with the knowledge
that the Colorado River
has carved the Grand Canyon.
[thunder]
>> The high plateau was
surrounded by even higher
mountain ranges.
New rivers began flowing
from the mountains
out across the plateau.
It's essential for
the investigation to establish
when the Colorado River,
in particular, arrived,
because only then could it
begin to carve the canyon.
Until a few decades ago,
some investigators thought
this ancient riverbed
called Hindu Canyon
provided the answer.
They believed that Hindu
Canyon's creation 50 million
years ago marked the arrival
of the Colorado River
and the beginnings
of the Grand Canyon.
But in 1969, the discovery
of these pebbles turned
everything that geologists
thought they knew about
the canyon on its head.
>> It turns out that to explain
how the Grand Canyon got there
is very much more complex
than people thought.
So the early geologists
thought it was simple,
but now we realize there's
a lot more to the story, and
it's kind of a detective story.
You start out with a few clues,
and you put the clues together,
and then finally you get
the satisfaction of saying,
"well, you know, I figured this
out before anybody else did."
>> Figuring it out
before anyone else was just
what Young did in 1969,
when he was a 24-year-old
geology graduate student
at Washington University.
His professor sent him
to investigate Hindu Canyon.
But when Toung arrived
at the dusty riverbed,
he discovered that it had
nothing to do with the Colorado
or the Grand Canyon itself.
His discovery flew in the face
of all the established
geological theories
and revolutionized thinking
about the canyon's history.
The evidence Young had uncovered
was the alignment of pebbles
in the bed of the river.
>> Look at these pebbles.
You can see that the pebbles
are flowing, or the pebbles
are arrayed in this direction,
which is a stable direction
for water flowing to my right.
If the pebbles had been
oriented this way, the water
would have flipped them over.
So when we find pebbles
that are oriented this way,
that tells us that the water
was flowing to my right.
>> It is a crucial clue.
The Colorado River could never
have flowed to Young's right.
It has always run
in the opposite direction,
towards the Pacific Ocean.
The river here, 50 million
years ago, was not the Colorado,
and it did not cut
the Grand Canyon.
Young's findings meant
scientists had to rethink
all their ideas about when
the Colorado had arrived
on the plateau and about
the age of the canyon.
They started examining evidence
from another less-ancient site.
This is Muddy Creek,
near Lake Mead, Arizona,
just a few miles downstream
from where the Colorado River
exits the Grand Canyon today.
The underlying rocks prove
that this was once the site
of a vast freshwater lake.
>> The upper part of the Muddy
Creek formation is this nice
limestone, which formed
in a freshwater lake.
The water would have been
very clean. There would have
been lots of plants and animals
living in the water.
And as they sank to the bottom,
the calcium carbonate
in their shells would form
this limestone, which is
typically what forms
the limestone rock.
>> The limestone
is the calcified remains
of the creatures that once
lived in this lake.
Then 5.5 million years ago,
the animals all disappeared.
There were no shells
to make fresh limestone.
The only explanation
is that the animals died
5.5 million years ago
because that was the date
when the Colorado River
arrived here.
The river would have been
carrying masses of dirt
and rock sediment
from the fledgling Grand Canyon.
>> The water would have been
too muddy and dirty,
and limestone does not form
in dirty, silty, muddy water.
It's just incompatible.
The animals and plants
that live in such a lake
can't exist if there's a lot
of silt and mud in the water.
>> So the muddy death
of the lake gave geologists
a confirmed date for when
the Colorado arrived in Arizona
and commenced its excavations.
The Grand Canyon was born
a mere 5.5 million years ago.
The investigation has reached
a significant milestone.
It has discovered the age
of the canyon.
The angle at which pebbles lie
in ancient riverbeds reveal
that the Grand Canyon
is far younger than geologists
had previously ever suspected.
The limestone discovered
at Muddy Creek reveals the date
that the Colorado River
arrived on the plateau
and the true age of the canyon--
5.5 million years old.
Now geologists had
a new mystery to solve--
discovering why the Colorado
River took the path it did
some 5.5 million years ago
and why it carved a canyon
of such remarkable dimensions.
>> The investigation into
the history of the Grand Canyon
has uncovered a 1.7 billion
year old landscape that has
evolved from ancient mountains
through to the Colorado plateau.
5.5 million years ago,
the Colorado River began
carving out the Grand Canyon
from this plateau.
The question scientists
now had to answer was
what happened at that time
yo cause the river to dig deep
and carve out the canyon.
It is a debate that has been
going on for more than a century
and which continues today.
>> I got interested
in the Grand Canyon
when I went to school
and started studying,
and I heard about
the different ideas associated
with the Grand Canyon and was
just blown away when I found out
we did not understand
how the Grand Canyon formed.
That something as iconic
as the Grand Canyon wasn't
understood just seemed crazy
to me, and so I basically
decided to dedicate
a large portion of my life
to trying to figure it out.
>> One theory is that several
ancient rivers merged,
and their combined cutting power
started digging out the canyon.
Another assumes that the river
cut down into the plateau
as the land uplifted around it.
But John Douglass has
his own theory, one that has
gained respect among many
leading geologists since
he first published his ideas
in 2000. What Douglass calls
his "spillover theory"
seems to work well on paper.
>> Spillover is incredibly easy.
All it means is the Colorado
River poured into a basin.
When it poured into that basin,
it had to form a lake,
and this lake was huge.
All that lake had to do was rise
and spill across the plateau.
It poured down, cutting rapidly,
and over time, you would have
ended up the beginnings
of Grand Canyon very quickly.
>> At his college campus
in Phoenix, Douglass is building
a scale model experiment to see
if his spillover theory
actually works in real life.
He sculpts tons of dirt into
a model of the Colorado plateau.
Running faucets represent
the flow of the Colorado River
into the ancient lake.
>> Now we have our large lake.
The water's getting higher.
It's getting ready to spill
across. We have a tiny little
trickle of water pouring down
off the lake.
That little tiny trickle
of water doesn't seem like much,
but over time, that little bit
of water flowing down that steep
slope is going to gain energy,
it's going to start cutting,
making waterfalls that work
their way back. One waterfall
has now reached the lake.
You can see that we have
just released a significant
amount of water, much more water
than was previously
pouring down.
Now we have huge canyon-cutting.
Landslides are sloughing off
the side of the canyon walls
into the water, flushing it
downstream. The lake,
you can see that it's starting
to shrink in size.
That lake is getting lower.
And right there, you can see
that we have cut our own
small-scale version
of the Grand Canyon.
>> Douglass' experiment proves
that the spillover theory works
in miniature, but he needs
evidence to show that it could
have happened on an infinitely
bigger scale.
Douglass sets out in search
of a lake large enough
and old enough to be the source
of his spillover flood.
He has a prime suspect in mind.
This is the site of the ancient
Lake Bidahochi, 100 miles
to the east of the Grand Canyon.
And a clue here on the old
lakebed reveals how deep
this lake once was.
>> These green clays,
which indicate deep lake water,
this is the classic evidence
for the giant lake necessary
for the overflow explanation
of Grand Canyon.
>> These green deposits
are only created
in one specific environment.
>> To have green lake clays,
you need deeper water,
where there is little oxidation.
I think that's an indication
that the Colorado River
has arrived in this basin,
that it's made its way
from the Rocky Mountains
to this location, and this is
its basically stopover point
before it eventually spills
across to form the Grand Canyon.
>> Establishing the depth
that Lake Bidahochi once spread
over 20,000 square miles
and contained more than 3,000
cubic miles of water.
That makes it bigger
than Lake Michigan.
Douglas needs to date
the age of this lake.
For his spillover theory
to work, it has to be older
than the Grand Canyon,
more than 5.5 million years.
He unearths the proof he needs
in these deep-water fossils.
>> Ok. These fossil shells
are freshwater mollusks
maybe as young as 6 million
years old.
>> The dates match up.
The lake was here at the right
time to have spilled over
and begun cutting
the Grand Canyon.
Dating the fossils helped
confirm Douglass' belief.
But his search for more
evidence continues.
>> In reality, we're never
going to know how it formed
to 100% certainty, unless
someone builds a time machine.
By doing this kind of work,
all you're trying to do
is increase your level
of confidence on your ideas,
build up your case,
build up your evidence.
>> Building up his evidence
is exactly what Joel Pederson
is doing. He is using
the very latest technology
to prove exactly how fast
the Grand Canyon was carved.
He has found the evidence
he needs right here
at the very start
of the Grand Canyon.
>> Here at Lee's Ferry,
there are all of these gravels
that are evidence of where
the river has been in the past
and the path it has taken
during incision,
and amongst the gravels,
sometimes you see
these great lenses of sand,
and we can use the sand
to get an absolute date
on these deposits.
>> As the Colorado River
carved out the canyon,
it deposited more and more
of the gravel and sand debris
at this spot. Newer layers
buried older layers
over millions of years.
And geologists now have
instruments that can measure how
light has affected individual
atoms within the sand.
That reveals precisely
when each sand layer
was originally buried,
away from the light of the sun.
For the technique to be
accurate, it is essential
that the sample is not exposed
to daylight.
>> So here we can take
a metal tube, and we can
hammer it into the sand outcrop,
and in the metal tube then,
we'll get a sample of the sand,
and it'll stay out of sunlight,
and then we take it back
to a darkroom laboratory
and remove the sand,
still sheltered from light,
and then we can analyze
the optical properties of it
to get an absolute age.
In this case, the absolute age
would tell us when the river
was at this point
in the landscape.
>> Back at the lab, Pederson
compares multiple sand samples,
each from a different depth
in the canyon deposits.
Discovering the age of each
individual layer of sand
lets him estimate how rapidly
the river has been cutting
down through the rocks.
>> Here at Lee's Ferry,
we can use this last
half-a-million-year history
along with our absolute dates
and all the information we get,
and the rate of canyon-cutting
here is about 1,000 feet
per million years.
>> That's one foot
in every thousand years,
a little more than one inch
every century. It proves
that the entire 5,300-foot-deep
Grand Canyon could have
been cut in a little more
than 5 million years.
In geological terms,
the mere blink of an eye.
The investigation is assembling
evidence on how the Grand Canyon
was created.
Green clay deposits support
the theory that an ancient lake
was big enough to spill over and
trigger the canyon's creation.
Deep-water fossils prove
that the lake existed
6 million years ago, the right
time to have overflowed
and cut the Grand Canyon.
But proving how
the canyon-cutting began is
only part of the investigation.
This is one of the widest
and deepest canyons
in the entire world,
and to discover how it grew
so large, geologists will
have to examine some of the most
dramatic and dangerous features
the canyon has to offer.
>> The landscape of western
Arizona has transformed
from ancient mountains
to prehistoric seas
to a flat uplifted plain.
Just 5.5 million years ago,
the Grand Canyon was cut
through this plateau.
It happened so fast that
geologists had to think again
about the awesome power
of the Colorado River.
>> It cuts through rock not by
yhe water wearing it away.
You could pour water over rock
for a long time, and nothing
would happen. It's the tools
that the river carries.
The river carries boulders
and sand, and those bump
against each other,
and they eat away at the rock.
>> Every day, the Colorado
can carry almost 500,000 tons
of rock and debris,
enough material to fill more
than 100 olympic swimming pools.
That is 5 tons every second.
So investigating river erosion
is never an easy task,
because the powerful flow
of the Colorado River
has scoured and washed away
many of the clues that
the rock detectives need.
They looked for evidence
in the hundreds of rapids that
disrupt the river's progress.
The swirling rapids are created
when flash floods sweep boulders
into the river from the many
smaller side canyons.
> The river has to focus
a lot of energy at these points
to deal with all of the boulders
that are coming in it.
The more coarse boulders
and more resistant material that
the river has to fight against
to accomplish its incision,
the steeper it gets.
>> As the water flows
over the rapids, it cuts deep
into the bedrock below.
At this set of rapids alone,
yhe river drops 10 feet,
and there are a lot of rapids
on this section of the Colorado.
>> So put all these rapids
together in a string
through Grand Canyon,
and that gives you the overall
sort of unusually steep
Grand Canyon profile
of the river going through it.
>> Gravity and simple physics
are at the heart of how
the Colorado has carved
so much rock so quickly.
5.5 million years ago,
the Colorado River was flowing
over the steep edge
of the plateau that had been
pushed up thousands of feet
above sea level.
The river ran rapidly
over the steep downward slope
and swept rough, rocky debris
along in its wake.
An incision formed, digging back
into the edge of the plateau.
>> The power of the river
to incise as it dropped off
that huge escarpment must have
been really great.
And so the river quickly incised
there, and that very steep
drop-off of incision would have
worked its way back upstream
through the Grand Canyon region.
It's sort of a waterfall that,
a few million years later,
has spread itself out
through the length of the river.
>> It is this steep drop-off
between the Colorado plateau
and the land beneath it
that fuels the Colorado's
incredible erosive power.
The river begins its life
high in the Rocky Mountains
in Colorado. For every mile
that it travels, the river
falls 10 feet. By contrast,
the Mississippi River, a river
moving 10 times as much water
as the Colorado, meanders
across a flat landscape.
With no steep slope to drive it,
the Mississippi can't carve
any canyons.
But the Grand Canyon
is not just steep.
It's also wide.
And here on the south rim,
at the heart of the National
Park, the true majesty
of the Grand Canyon is revealed.
This is where the canyon
is at its widest, as much
as 18 miles from rim to rim.
This landscape appears serene
now, but its unique beauty
was forged by violent forces.
>> Grand Canyon, oftentimes
we just associate it
with the Colorado River.
The Colorado River is what cut
the Grand Canyon. It formed
the initial gash to allow
the river to flow across.
But what makes Grand Canyon
grand is really its width.
And all the layers of rock
that are exposed, and that
isn't only solely tied
to the Colorado River.
What's happening is this rock
that's exposed, it's being
beaten on by rain, and the rain
gets in there, and it weathers
the rock, and it weakens it.
And then because this is
so incredibly steep, gravity
will act on that material,
transporting it deeper
down into the river,
flushing it back out.
And that process just repeats
over and over again
to allow the canyon
yo get wider over time.
We have classic rockfalls
that are cascading down
onto the black rock
in the far distance.
Those events are indications
that this is actively ongoing
canyon-widening and retreating
from these processes.
>> The fall of these rocks
is not a gradual process.
This is erosion
at its most violent.
>> Very few people are ever
going to see Grand canyon
actually change. I've spent
numerous nights in Grand Canyon.
I've only heard one or two rocks
ever fall, but change will
happen, and when it does happen,
it happens very rapidly.
>> The rocks fall because both
harder and softer rocks
are layered, one on top of
the other, in the canyon walls.
The harder layers are made
of limestone and sandstone.
These rocks don't weather
easily, but the softer shale
beneath is made of mud that
expands when it rains, causing
the shale to crumble away.
>> Those weaker rocks,
yhey weather and retreat back,
and they undermine
the resistant cliff rocks above
that will then fail as dramatic
rockfall, landslide events,
allowing the canyon
to increase its width.
>> The rockfalls are merely
the first step towards
increasing the Grand Canyon's
enormous width. Without the help
of an accomplice,
the entire canyon would
fill up with debris.
>> Without the Colorado River,
you could not have the Grand
Canyon as wide as it is.
By flushing that material
downstream, it like...
it wipes it all clean
to allow a whole new material
to build up again.
And once you repeat that
over and over again,
it allows the canyon walls
to retreat back, and the entire
canyon just grows, as these
guys continue to march and push
and move all that material
downstream.
>> The mystery of how the Grand
Canyon grew so deep and so wide
is being solved.
The Colorado rapids demonstrate
how the steepness of the
riverbed helps carve
the canyon so quickly.
Rockfalls on the canyon walls
reveal how weaker rocks
rapidly widen the canyon
across the plains of Arizona.
But this is far from the end
of the Grand Canyon's story.
In just the last million years,
the canyon has been transformed
by other overwhelmingly powerful
natural forces.
>> Geologists have established
that over 1.7 billion years,
the Grand Canyon emerged
from ancient mountains
and prehistoric seas to become
one of North America's
geological icons.
This is a rare look at one
of the most remote and secret
parts of the Grand Canyon.
A series of small cone-shaped
mountains line the canyon's
edge.
And there are flows
of black rock running down
from each rim.
They come from a remarkable era
just 725,000 years ago,
when the peace of the canyon
was shattered...
[rumbling]
by volcanoes.
This is Toroweap Point,
in a remote area known
as the Arizona strip...
one of the most isolated places
in the continental U.S.
Few people, other than
geologists, ever see this area,
although it boasts some of the
canyon's most stunning views.
The rock detectives come to see
how explosive volcanic
eruptions have changed
the canyon in the comparatively
recent geological past.
This black rock that seems
to have spilled over the rim
of the canyon is an ancient
lava flow, what was once
boiling-hot rock,
Forever frozen in time.
>> Powell talked about a river
of molten magma pouring down
into a river of melted snow.
And he talked about
how dramatic it must have been,
the boiling and seething
and the steam, and...
it must have been amazing.
You would picture red-hot lava
like you would see in Hawaii
pouring down the canyon walls
and coating them, and then
once it reached the river,
it would... you know, it would
immediately create just giant
clouds of steam.
>> The extensive lava flows
erupting from as many as 100
cinder-cone volcanoes
had a dramatic effect
on the Colorado River
running below.
Crow believes that on at least
8 occasions, the volcanic
eruptions created huge
lava dams that blocked
the river completely.
>> Well, behind me here
is one of many basalt remnants.
They're the remains of lava
flows that poured down the
canyon, partially filling it.
And then subsequently,
the Colorado River has removed
all but a few little chunks.
>> The lava dams brought
even the powerful Colorado River
to a halt...
For a while.
In time, the dams were no match
for the Colorado.
The rising pressure
of the dammed river behind them
eventually became too much,
and they shattered.
This explosive episode
has left its mark
on the canyon's walls.
Today, the cones appear
to be extinct and lifeless,
although some geologists believe
that the volcanoes might not
be finished quite yet.
>> The last eruption that sent
lava pouring into Grand Canyon
probably occurred about
100,000 years ago. There is
evidence for an eruption
on the rim that didn't actually
make it into Grand Canyon
that's 1,000 years old.
So there's... there's, you know,
I think, a good chance
that in the future, there may
be eruptions here as well.
>> The Grand Canyon's future
has yet to be written,
but investigators now understand
the story of its past.
The calcium in the garnet
discovered at the base
of the canyon reveals
the ancient beginnings
of this landscape--
an immense mountain range.
Limestone rocks show
that the canyon was only formed
5.5 million years ago.
Green clays that can only form
in deep water prove that
a huge lake, bigger than
Lake Michigan, could have been
the trigger for this
canyon-carving.
And rockfalls from
the crumbling cliff faces
of the canyon rim are evidence
of how the canyon grew
to the shape it is today.
Geologists have been studying
the canyon since the mid-1800's.
Yet even after more than
a century of investigation,
the story is still
far from finished.
>> The landscape is evolving,
and it's going to be changing
through the geological future.
And so the story about
the geology and the fascinating
questions here is not one
that's over, and it's going to
continue to evolve as scientists
continue to do work here.
>> The dynamic geological
phenomenon of the Grand Canyon
is a place where the vast
fiery forces within the Earth's
crust do battle with
the inexorable power of water.
The result--a natural wonder
whose walls record
nearly 2 billion years
of our planet's turblent
geological history.
-- Sync, corrected by elderman -- -- for www.addic7ed.com --